Grade crossings, where automobile traffic crosses railroad tracks, have been a notorious site for collisions. Various types of warning systems to warn road traffic of the approach of a train, rely on two major warning sources: (1) an audible signal from a locomotive horn, and (2) at least a visual indicator of the presence of the railroad crossing.
While the visual indicator at the railroad crossing varies from a pair of "cross-bucks" to fully automated crossing gates with lights and bells, the second part of the equation continues to rely on the timely occurrence of horn blasts from the locomotive. Since the driver of the automobile must have sufficient time to stop at the crossing in response to a warning signal, the horn blast from the locomotive must occur at a sufficient distance from the automobile driver. In addition, to produce a sound sufficiently loud to be heard by the automobile driver, while the locomotive is still approaching the intersection, the horn blast must be activated at a very high decibel level.
Obviously, the main problem associated with horn blasts on a locomotive is the disturbance to residents of an urban area located adjacent to the crossing. A related, but less common problem, occurs when a train is backing over a crossing, wherein the horn is located on the opposite end of the train.